The success of rotary drilling enabled the discovery of deep oil and gas reservoirs. The rotary rock bit was an important invention that made rotary drilling economical.
Only soft earthen formations could be penetrated commercially with the earlier drag bit, but the two-cone rock bit, invented by Howard R. Hughes, U.S. Pat. No. 930,759, drilled the hard caprock at the Spindletop Field near Beaumont, Tex., with relative ease. That venerable invention, within the first decade of this century, could drill a scant fraction of the depth and speed of the modern rotary rock bit. If the original Hughes bit drilled for hours, the modern bit drills for days.
Modern bits sometimes drill for thousands of feet instead of merely a few feet. Many advances have contributed to the impressive improvement of rotary rock bits.
In drilling boreholes in earthen formations by the rotary method, rock bits fitted with one, two, or three rolling cutters are employed. The bit is secured to the lower end of a drillstring that is rotated from the surface or by downhole motors or turbines. The cutters mounted on the bit roll and slide upon the bottom of the borehole as the drillstring is rotated, thereby engaging and disintegrating the formation material to be removed. The roller cutters are provided with teeth or cutting elements that are forced to penetrate and gouge the bottom of the borehole by weight from the drillstring. The cuttings from the bottom and sidewalls of the borehole are washed away by drilling fluid that is pumped down from the surface through the hollow, rotating drillstring are carried in suspension in the drilling fluid to the surface. The drilling fluid discharge onto the bottom and sidewalls of the borehole through nozzles or jets carried by the bit.
Typically, the drilling fluid is a liquid (water or oil) with a solid material in suspension. Liquid drilling fluid or mud circulates in the borehole to cool and lubricate the bit, wash away cuttings, protect the uncased formation against sloughing and caving, and to provide a hydrostatic pressure column in the borehole to counteract pressure imbalances in the borehole.
Air or a gaseous drilling fluid is known to permit high rates of penetration when it can be used. However, because of its reduced density and tendency to form explosive mixtures with natural gas in the borehole, air or gas is not used as a drilling fluid in most applications.
The design and arrangement of the nozzles or jets of a bit has been the subject of a good deal of study. In general terms, the nozzles can be designed to discharge fluid primarily for one of two purposes: to maximize cleaning of the cutting structure of the bit and washing of cuttings from the bottom of the borehole, or to impinge directly upon the bottom and sides of the borehole in an attempt to aid the bit cutting structure in disintegrating formation material.
Under certain drilling conditions, liquid drilling mud systems have the effect of actually confining cuttings at the bottom of the borehole, where they are reground to a very fine consistency and contribute to bit balling and abrasive wear of the components of the bit.